Reflection liquid crystal display element

ABSTRACT

In a reflective liquid crystal display device with a single polarizing film and two retardation films located in between the polarizing film and liquid crystal cell, the angle of twist of nematic liquid crystal is 45°˜90°, the product of the birefringence of the liquid crystal and the thickness of the liquid crystal layer is 0.20 μm˜0.30 μm, the retardation value of the retardation film on the polarizing film side is 0.13 μm˜0.18 μm, the retardation value of the retardation film on the liquid crystal cell side is 0.13 μm˜0.18 μm, and the Z coefficient of each of two retardation films is 0.3˜1.0. This invention is featured by the following optical relations, that is, Ø P  75°˜195°, Ø P −Ø F1 , 105°˜115°, and Ø P −Ø F2 , 165°˜175°, where Ø P , Ø F1  and Ø F2  are angles formed by the reference line and respectively the direction of the absorption axis of the polarizing film, the direction of the slow axis of the retardation film on the polarizing film side, and the direction of the slow axis of the retardation film on the liquid crystal cell side. With the foregoing construction, a high contrast display of bright achromatic white and achromatic black is achieved.

TECHNICAL FIELD

The present invention relates to a reflective liquid crystal displaydevice with a reflection electrode.

BACKGROUND OF THE INVENTION

Liquid crystal display devices (LCD) are thin and light thus have beenused for a wide range of purposes such as a display for a mobileinformation terminal. LCDs do not emit light themselves. They are alight-receiving-type device which displays an image by changing thelevel of transmittance of light. LCDs can be driven by effective voltageof several volts. Therefore, if LCD is used as a reflective-type bydisposing a reflecting plate under the LCD to utilize reflection ofexternal light, no electricity is required for a back light, thus thedisplay device becomes extremely efficient in terms of powerconsumption.

A conventional reflective color LCD comprises a liquid crystal cellincluding color filters, and a pair of polarizing films which sandwichthe liquid crystal cell. The color filters are disposed on one ofsubstrates of the liquid crystal cell. On top of the color filtersdisposed on the substrate is a transparent electrode. By applyingvoltage on the liquid crystal cell, orientation of the liquid crystalmolecules is changed, thereby controlling the transmittance of light ofeach color filter to display a color image.

The transmittance of a polarizing film is only about 45% at the maximum.The transmittance of the polarized light parallel to the absorption axisof the polarizing film is approximately 0% while that of the polarizedlight perpendicular to the absorption axis of the polarizing film is90%. In the case of the reflective LCD using two polarizing films, lightgoes through polarizing films four times before emitted. Therefore, themaximum reflectance can be defined as:

 (0.9)⁴×50%=32.8%

when absorption by other materials such as a color filter is notconsidered.

Thus, even the reflectance of a black and white display panel which doesnot use color filters is only about 33%. If color filters are applied tosuch a device to display a color image, the reflectance will be reducedto about one third of the original reflectance, inhibiting theachievement of a reflectance high enough for a sufficient luminance.

To brighten the display, several constructions have been proposed, forexample, by the Japanese Patent Application Unexamined Publications No.H07-146469 and No. H07-84252, in which only one polarizing film is usedon top of the liquid crystal cell, which is sandwiched between thepolarizing film and a reflecting plate. In this case, light goes throughthe polarizing film only twice, thus the maximum reflectance can bedefined as:

(0.9)²×50%=40.5%

when absorption by other materials such as the color filters is notconsidered.

Therefore, in this case, a maximum increase of 23.5% in reflectance fromthe construction using two polarizing films can be expected.

However, with this reflective LCD with one polarizing film, whendisplaying a color image by using color filters while trying to obtainhigher luminance by increasing the reflectance, color drift oftenoccurs, obstructing a clear achromatic display. In particular, anachromatic black has been difficult to display.

The Japanese Patent Application Unexamined Publication No. H06-308481has disclosed a reflective color LCD which displays a colored imagewithout color filters by utilizing birefringence of the twisted nematicliquid crystal layer and polarizing films. The Japanese PatentApplication Unexamined Publications No. H06-175125 and No. H06-301006have disclosed a color LCD which utilizes birefringence of a liquidcrystal layer and phase retardation films. Since these LCD do not usecolor filters, a reflectance high enough to achieve practical luminanceis ensured even when two polarizing films are used. However, in the caseof the foregoing devices, since a colored image is displayed based oncoloring by birefringence, multi-gradation, multi-color displays such as16 gradation, 4096 color display and 64 gradation full-color displayhave principally been difficult. Moreover, the color purity and thecolor reproduction range have been limited.

Considering the aforementioned issues, the present invention aims atproviding a reflective LCD that achieves a bright white display and ahigh contrast, and is capable of displaying achromatic black and whiteas well as multi-gradation and multi-color displays.

SUMMARY OF THE INVENTION

To achieve the foregoing purpose, the present invention has thefollowing construction.

The reflective liquid crystal display device of a first construction ofthe present invention comprises the following elements;

a) a liquid crystal cell formed of a nematic liquid crystal injectedbetween a first and a second substrates;

b) a polarizing film disposed on the first substrate of the liquidcrystal cell;

c) two retardation films disposed in between the polarizing film and theliquid crystal cell; and

d) a light reflecting means disposed on the second substrate. The twistangle of the nematic liquid crystal Δ_(LC) is 45°˜90°. The product ofthe birefringence of the nematic liquid crystal Δn_(LC) and thethickness of the liquid crystal layer d_(LC), namely Δn_(LC)·d_(LC) is0.20 μm˜0.3 μm. The retardation value of the retardation film on thepolarizing film side R_(F1) is 0.23 μm˜0.28 μm. The retardation value ofthe retardation film on the liquid crystal cell side R_(F2) is 0.13μm˜0.18 μm. The Z coefficient Q_(Z) of each retardation film is 0.3˜1.0.When bisector of the larger of the angle formed by the orientationdirection of the liquid crystal molecules closest to one of thesubstrates and the orientation direction of the liquid crystal moleculesclosest to the other substrate, is set as a reference line in thesubstrate, and when the direction in which the nematic liquid crystal istwisted from the first substrate to the second substrate, viewed fromthe side of the first substrate, is defined positive, the presentinvention is featured by

Ø_(P) is 75°˜195°, Ø_(P)−Ø_(F1), 105°˜115°, and Ø_(P)−Ø_(F2), 165°˜175°,

where an angle formed by the reference line and the direction of theabsorption axis of the polarizing film is Ø_(P), an angle formed by thereference line and the direction of the slow axis of the retardationfilm on the polarizing film side is Ø_(F1), and an angle formed by thereference line and the direction of the slow axis of the retardationfilm on the liquid crystal cell side is Ø_(F2).

The aforementioned Q_(Z) is a coefficient defined as:

Q _(Z)=(n _(x) −n _(z))/(n _(x) −n _(y))

where n_(x), n_(y) and n_(z) are refractive indices of each retardationfilm in the directions of each axis of spatial coordinates (x, y, z) inwhich the z axis is a direction normal to the retardation film, the xand y axes are parallel to respectively the slow axis and the fast axisof each retardation film. n_(x) is a refractive index in the directionof the slow axis and n_(y) the fast axis of each retardation film.

According to the first construction, a bright normally-white reflectiveliquid crystal display device displaying achromatic black and white canbe achieved.

In the case of the first construction, the angle Ø_(P) formed by thereference line and the absorption axis of the polarizing film ispreferably 90°˜120° or 150°˜180°. This preferable construction achieveseven higher contrast and better display properties.

A reflective liquid crystal display device of a second construction ofthe present invention is based on the first construction and is featuredby

Ø_(P) is−15°˜105°, Ø_(P)−Ø_(F1), −105°˜−115°, and Ø_(P)−Ø_(F2),−165°˜−175°.

According to the second construction, a bright normally-white reflectiveliquid crystal display device, also displaying achromatic black andwhite can be achieved.

In the case of the second construction the angle Ø_(P) formed by thereference line and the absorption axis of the polarizing film ispreferably 0°˜30° or 60°˜90°. This preferable construction achieves evenhigher contrast and better display properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing the constructionof a reflective LCD in accordance with a first preferred embodiment ofthe present invention.

FIG. 2 is an optical schematic diagram of a reflective liquid crystaldisplay device in accordance with preferred embodiments of the presentinvention.

FIG. 3 is a graph showing a relationship between reflectance of thereflective liquid crystal display device and the applied voltages inaccordance with the first preferred embodiment of the present invention.

FIG. 4 is a cross-sectional view schematically showing the constructionof a reflective liquid crystal display device in accordance with asecond preferred embodiment of the present invention.

FIG. 5 is a graph showing a relationship between reflectance and theapplied voltages of the reflective liquid crystal display device inaccordance with a third preferred embodiment of the present invention.

FIG. 6 is a graph showing the reflectance according to the shift inviewing angles in the right direction caused by the difference in Zcoefficient.

FIG. 7 is a graph showing the reflectance according to the shift inviewing angles in the right direction caused by the difference in Zcoefficient.

FIG. 8 is a graph showing the reflectance according to the shift inviewing angles in the right direction caused by the difference in Zcoefficient.

FIG. 9 is a graph showing the reflectance according to the shift inviewing angles in the right direction caused by the difference in Zcoefficient.

FIG. 10 is a cross-sectional view schematically showing the constructionof a reflective liquid crystal display device in accordance with a fifthpreferred embodiment of the present invention.

FIG. 11 is a cross-sectional view schematically showing the constructionof a reflective liquid crystal display device in accordance with a sixthpreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings.

The First Preferred Embodiment

FIG. 1 is a cross-sectional view schematically showing the reflectiveLCD in accordance with the first preferred embodiment. As shown in FIG.1, the device in this embodiment includes a liquid crystal cell 1sandwiched between an upper transparent substrate 13 and a lowersubstrate 19, and on top of the laminate, a scattering film 12, tworetardation films 11 a and 11 b made with polycarbonate film, and apolarizing film 10 are disposed. On the inner face of the lowersubstrate 19, a metallic reflecting electrode 18 and an orientationlayer 15 b are layered, and on the inner surface of the uppertransparent substrate, a color filter layer 14, a transparent electrode16 and an orientation layer 15 a are disposed. A liquid crystal layer 17of a thickness of d_(LC) is injected in between the two substrates toform the liquid crystal cell 1.

FIG. 2 is an optical schematic diagram of a reflective LCD viewed from adirection normal to the plane in accordance with the first preferredembodiment. A direction, which bisects the larger of the angle formed bythe orientation directions 21 and 22 of liquid crystal moleculesrespectively closest to the lower substrate and to the upper transparentsubstrate, is used as a reference line 20. FIG. 2 shows a direction 23 aof the slow axis of the retardation film 11 a disposed on the polarizingfilm side, a direction 23 b of the slow axis of the retardation film 11b on the liquid crystal cell side, and a direction 24 of the absorptionaxis of the polarizing film. Angles Ø_(p), Ø_(F1) and Ø_(F2) are theangles respectively of the direction 24 of the absorption axis of thepolarizing film 10, the direction 23 a of the slow axis of theretardation film 11 a on the polarizing film side, and the direction 23b of the slow axis of the retardation film 11 b on the liquid crystalcell side measured from the reference line 20. With respect to the signof the angles, viewed from the upper transparent substrate side, thedirection, Ω_(LC), in which the liquid crystal molecules are twistedfrom the upper transparent substrate to the. lower substrate, is definedas positive.

The upper transparent substrate 13 and the lower substrate 19 are madewith non-alkali glass substrate (for example, product 1737 made byCorning Co.). On top of the upper transparent substrate 13 is the colorfilter layer 14 formed by the lithography method and which is apigment-dispersion type with red, green, and blue being arranged in astrip-form. The transparent electrode 16 is a pixel electrode made withan indium tin oxide, formed on the color filter layer 14. The metallicreflecting electrode 18 of a specular reflection type is formed on topof the lower substrate 19 by vapor-depositing 300 nm-thick titanium, and200 nm-thick aluminum thereupon.

A 5% by weight polyimide γ-butylolactone solution, is printed on thetransparent electrode 16 and the metallic reflecting electrode 18, thencured at 250° C. Subsequently, in an orientating process, a twist of apredetermined angle is provided by a rotation rubbing method using arayon cloth, to form the orientation layers 15 a and 15 b.

A thermosetting sealing resin (for example, Structbond made by MitsuiToatsu Chemical. Co.) containing 1.0% by weight of glass fibers of apredetermined diameter is printed in the vicinity of the inner face ofthe upper transparent substrate 13. Resin beads of a predetermineddiameter are dispersed at 100˜200/mm² on the inner face of the lowersubstrate 19. The upper transparent substrate 13 and the lower substrate19 are bonded together and the sealing resin is cured at 150° C.Subsequently, chiral liquid crystal is added to fluorine-ester nematicliquid crystal of the birefringence difference Δn_(LC)=0.08 in such amanner that the chiral pitch becomes 80 μm. The mixed liquid crystal isinjected under vacuum in between the two substrates, and the opening issealed with ultraviolet curing resin which is then cured by UV lights.

As the scattering film 12, isotropic front scattering film is layered onthe upper transparent substrate 13 of the liquid crystal cell 1 formedin the foregoing manner. The retardation films 11 a and 11 b are layeredon the scattering film 12 in such a way that their slow axes form anangle specified below. On top of these films, a neutral gray polarizingfilm (SQ-1852AP made by Sumitomo Chemical Industry Co.) undergoneanti-glare (AG) and anti-reflection (AR) treatments, is attached as thepolarization film 10 in such a manner that its absorption axis forms anangle specified below.

The retardation films include a film having positive uniaxialbirefringence anisotropy in the direction inside the surface of thefilm, and a film having biaxial birefringence anisotropy in directionsinside and perpendicular to the surface of the film. To show the levelof the birefringence anisotropy in the direction perpendicular to theface, Z coefficient, Q_(z) is used. Q_(Z) is a coefficient defined as:

Q _(Z)=(n _(x) −n _(z))/(n _(x) −n _(y))

where n_(x), n_(y) and n_(z) are refractive indices in the directions ofeach axis of spatial coordinates (x, y, z) in which the z axis is adirection normal to the retardation films, and n_(x) is a refractiveindex in the direction of the slow axis and n_(y) the fast axis of theretardation film. In the case of uniaxial film Q_(Z)=1.

Under the standard condition of this embodiment, the thickness of theliquid crystal is set at d_(LC)=3.0 μm, Δn_(LC)·d_(LC), equal to 0.24μm, and the twist angle of the liquid crystal, Δ_(LC)=63.0°. As the tworetardation films, films of Q_(Z)=1.0 are used. The retardation of theretardation film 11 a on the polarizing film side is set as R_(F1)=0.27μm whereas that of the retardation film 11 b on the liquid crystal cellside is set as R_(F2)=0.14 μm. Angles of their slow axes against theabsorption axis of the polarizing film are set as Ø_(P)−Ø_(F1)=110.0°and Ø_(P)−Ø_(F2)=170.0°.

The optical properties of the device prepared based on the foregoingconditions are measured in a reflection mode by changing the angle Ø_(P)of the absorption axis of the polarizing film. The result indicates thatwhen Ø_(P) is in the range of 75°˜195°, a normally-white mode reflectiveLCD with a high contrast can be achieved. The reason for this is thatwhen the absorption axis of the polarizing film is set at this angle,the luminance of black can be lowered.

When Ø_(P) is in the range of 90°˜120° or 150°˜180°, a normally-whitemode reflective LCD achieving a high contrast display of achromaticblack and white can be obtained, thus it is especially preferable.

With regard to the properties measured by changing Δn_(LC)·d_(LC), whenits value is in the range of 0.20 μm˜0.30 μm, achromatic black low inreflectance and achromatic white high in reflectance are obtained.

The properties measured by changing the twist angle Δ_(LC) of the liquidcrystal suggest that, in this embodiment, favorable properties areobtained when the twist angle Δ_(LC) is in the range of 45°˜90°.Especially favorable properties are obtained when the twist angle Δ_(LC)is in the range of 60°˜65°.

When the values of Ø_(P)−Ø_(F1) and Ø_(P)−Ø_(F2) are not in theaforementioned range, the contrast is lowered. Favorable ranges are105°˜115° for Ø_(P)−Ø_(F1), and 165°˜175° for Ø_(P)−Ø_(F2). The value ofthe retardation of each retardation film has its favorable range; RF is0.23˜0.28 μm, and R_(F2), 0.13˜0.18 μm. In these ranges, favorablecontrast can be achieved.

The optical properties measured when Ø_(P) is set at 105.0° based on theforegoing conditions are described below. The measurement of thereflectance is conducted against a perfect diffuse light source.

FIG. 3 is a graph showing a relationship between reflectance of thereflective LCD and the applied voltages in accordance with the firstpreferred embodiment. Here the reflectivity is expressed in terms ofY-value in the XYZ colorimetric system as obtained by converting thebrightness of white displayed on the liquid crystal display devicerelative to the reflectivity of a standard white panel being defined as100%. As for the front properties (which are the properties measuredfrom the direction normal to the display surface), the reflectance ofwhite converted to a value of Y is 19.5%, and the contrast is 15.9. Thereflectance changes from black to white achromatically, thus64-gradation, full color display can be achieved.

When a reflective LCD is produced without the color filter layer 14according to the foregoing construction, a contrast of 15.5 and thereflectance of white converted to a value of Y of 35.3% in terms offront properties can be achieved.

In the foregoing description, the scattering film 12 is sandwichedbetween the retardation film 11 b and the upper transparent substrate13, the same properties can be achieved when the scattering film 12 isdisposed on top of the polarizing film 10, and when it is sandwichedbetween the retardation film 11 a and 11 b, the same properties can beachieved.

The Second Preferred Embodiment

FIG. 4 is a cross section schematically showing the construction of areflective LCD in accordance with the second preferred embodiment. Theelements having the same functions as those of the first preferredembodiment shown in FIG. 1 are denoted by the same numerals. Thisembodiment differs from the first preferred embodiment in the followingways:

the scattering film 12 is not used; and

instead of the metallic reflecting electrode 18 of a specular reflectiontype, a metallic reflection electrode 48 of a scatter reflection type isused.

The metallic reflection electrode 48 of a diffusion (scattering)reflection type is formed by vapor-depositing 300 nm-thick titanium andthereupon, 200 nm-thick aluminum on the lower substrate 19. The surfaceof the aluminum is made uneven so that it has angles of slopes 3°˜12° onaverage. The rest of the manufacturing method of the display device isnot explained here since it is the same as that of the first preferredembodiment.

The optical construction of this embodiment is the same as that of thefirst preferred embodiment, namely, the same as the optical constructionof the reflective LCD shown in FIG. 2.

In this embodiment as well, the thickness of the liquid crystal is setat d_(LC)=3.0 μm, Δn_(LC)·d_(LC), equal to 0.24 μm, and the twist angleof the liquid crystal, Δ_(LC)=63.0°, as the standard condition. As thetwo retardation films, films of Q_(Z)=1.0 are used. The retardation ofthe retardation film 11 a on the polarizing film side is set asR_(F1)=0.27 μm whereas that of the retardation film 11 b on the liquidcrystal cell side is set as R_(F2)=0.14 μm. Angles of their slow axesagainst the absorption axis of the polarizing film are set asØ_(P)−Ø_(F1)110.0° and Ø_(P)−Ø_(F2)170.0°.

The optical properties of the device prepared based on the foregoingconditions are measured in a reflection mode by changing the angle Ø_(P)of the absorption axis of the polarizing film. The result indicates thatwhen Ø_(P) is in the range of 75°˜195°, a normally-white mode reflectiveLCD with a high contrast can be achieved. The reason for this is thatwhen the absorption axis of the polarizing film is set at this angle,the luminance of black can be lowered.

When Ø_(P) is in the range of 90°˜120° or 150°˜180°, a normally-whitemode reflective liquid crystal display device with a high contrastdisplay of achromatic black and white can be obtained, thus it isespecially preferable.

With regard to the properties measured by changing ΔA_(LC)·d_(LC), whenits value is in the range of 0.20 μm˜0.30 μm., achromatic black low inreflectance and achromatic white high in reflectance are obtained.

The properties measured by changing the twist angle Δ_(LC) of the liquidcrystal suggest that, in this embodiment, favorable properties areobtained when the twist angle Δ_(LC) is in the range of 45°˜90°.Especially favorable properties are obtained when the twist angle Δ_(LC)is in the range of 60°˜65°.

The following is the result obtained when the optical properties aremeasured by setting Ø_(P) at 105.0° based on the foregoing conditions.The measurement is conducted against the perfect diffuse light source.

In terms of the front properties, the reflectance of white converted toa value of Y of 18.8%, and the contrast of 15.6 are achieved. Thereflectance changes achromatically from black to white, thus, it isconfirmed, a 64-gradation, full color display can be achieved.

When a reflective LCD is manufactured by removing the color filter layer44, a contrast of 15.3, and the reflectance of white converted to avalue of Y 34.1% in terms of front properties, are achieved.

The Third Preferred Embodiment

The manufacturing method and construction of the reflective LCD of thethird preferred embodiment are the same as those of the first preferredembodiment, as such, the reflective LCD of this embodiment has the crosssection shown in FIG. 1 and the optical construction of FIG. 2.

In this embodiment, the angles of the slow axes of the retardation filmsare set differently from the previous embodiments.

In this embodiment as well, under the standard condition, the thicknessof the liquid crystal is set at d_(LC)=3.0 μm, and Δn_(LC)·d_(LC), equalto 0.24 μm, and the twist angle of the liquid crystal, Ω_(LC)=63.0°. Asthe two retardation films, films of Q_(Z)=1.0 are used. The retardationof the retardation film 11 a on the polarizing film side is set asR_(F1)=0.2 μm whereas that of the retardation film 11 b on the liquidcrystal cell side is set as R_(F2)=0.14 μm.

Angles of their slow axes of the retardation films against theabsorption axis of the polarizing film are set as Ø_(P)−Ø_(F1)=110.0°and Ø_(P)−Ø_(F2)=−170.0°.

The optical properties of the device prepared based on the foregoingconditions are measured in a reflection mode by changing the angle ØP ofthe absorption axis of the polarizing film. The result indicates thatwhen Ø_(P) is in the range of −15°˜105°, a normally-white modereflective LCD with a high contrast can be obtained. The reason for thisis that when the absorption axis of the polarizing film is set at thisangle, the luminance of black can be lowered.

When the values of Ø_(P)−Ø_(F1) and Ø_(P)−Ø_(F2) are not in theaforementioned range, the contrast is lowered. Favorable ranges are−105°˜−115° for Ø_(P)−Ø_(F1), and −165°˜−175° for Ø_(P)−Ø_(F2). Thevalue of the retardation of the retardation film has its favorablerange; R_(F1) is 0.23˜0.28 μm, and R_(F2), 0.13˜0.18 μm. In theseranges, favorable contrast can be achieved.

When Ø_(P) is in the range of 0°˜30° or 60°˜90°, a normally-white modereflective LCD with a high contrast display of achromatic black andwhite can be obtained, thus it is especially preferable.

With regard to the properties measured by changing Δn_(LC)·d_(LC), whenits value is in the range of 0.20 μm˜0.30 μm., achromatic black low inreflectance and achromatic white high in reflectance are obtained.

The properties measured by changing the twist angle Ω_(LC) of the liquidcrystal suggest that, in this embodiment, when the twist angle Ω_(LC) isin the range of 45°˜90° favorable properties are obtained. Especiallyfavorable properties are obtained when the twist angle Ω_(LC) is in therange of 60°˜65°.

The following is the result obtained when the optical properties aremeasured by setting Ø_(P) at 75.0° based on the foregoing conditions.The measurement is conducted against the perfect diffuse light source.

FIG. 5 is a graph showing a relationship between reflectance and theapplied voltages of the reflective LCD in accordance with the thirdpreferred embodiment. As for the front properties, the reflectance ofwhite converted to a value of Y is 19.3%, and the contrast is 15.8. Thereflectance changes achromatically from black to white, thus it isconfirmed that a 64-gradation, full color display can be achieved.

When a reflective LCD is manufactured without the color filter layer 14,contrast of 15.3, and the reflectance of white converted to a value of Yof 35.1% are achieved in terms of front properties.

In the foregoing description, the scattering film 12 is sandwichedbetween the retardation film 11 b and the upper transparent substrate13, however, the same properties can be achieved when the scatteringfilm 12 is disposed on top of the polarizing film 10, or when it issandwiched between the retardation film 11 a and 11 b.

The Fourth Preferred Embodiment

The manufacturing method and construction of the reflective LCD of thethird preferred embodiment are the same as those of the first preferredembodiment, as such, the reflective LCD of this embodiment has the crosssection shown in FIG. 1 and the optical construction of FIG. 2. In thisembodiment, however, films of biaxial birefringence anisotropy, namelyfilms having different Z coefficient, Q_(Z) from those of the firstpreferred embodiment are used as the retardation films.

In this embodiment as well, under the standard condition, the thicknessof the liquid crystal is d_(LC)=3.0 μm, Δn_(LC)·d_(LC), equal to 0.24μm, and the twist angle of the liquid crystal, Ω_(LC)=63.0°. Theretardation of the retardation film 11 a on the polarizing film side isset as R_(F1)0.27 μm whereas that of the retardation film 11 b on theliquid crystal cell side is set as R_(F2)=0.14 μm.

Angles of the slow axes of the retardation films against the absorptionaxis of the polarizing film are set as Ø_(P)−Ø_(F1)=−110.0° andØ_(P)−Ø_(F2)=−170.0°.

In this embodiment, the angle of the absorption axis of the polarizingfilm is fixed at Ø_(P)=105.0°. The optical properties of the deviceprepared based on the foregoing conditions are measured in a reflectionmode by changing the Z coefficient, Q_(Z) (1) of the retardation film 11a on the polarizing film side and the Z coefficient, Q_(Z) (2) of theretardation film 11 b on the liquid crystal cell side. The resultsuggests that when both Q_(Z) (1) and Q_(Z) (2) are 0.3˜1.0, favorableproperties with reduced changes in reflectance, contrast and coloraccording to the different viewing angles can be achieved.

Changes in reflectance of four gradations from white to black whenapplying voltage, according to the viewing angles, are measured on fourcombinations of Q_(Z) (1) and Q_(Z) (2) being either 0.5 or 1.0.

FIGS. 6˜9 are graphs in which changes in reflectance against the viewingangles shifting from the front (normal to the surface) of the panel(Θ=0) to the right in FIG. 2 by angle Θ, are plotted. In other words,FIGS. 6˜9 show reflectance dependence on the viewing angle shifting tothe right.

As shown in FIGS. 6˜9, the smaller the Z coefficient, Q_(Z) is, thebetter are the reflectance properties with a reduced dependency onviewing angles and no tone reversal. Favorably, Q_(Z) (1) is in therange of 0.3˜0.7. Further, it is confirmed that even better viewingangle properties can be achieved when both Q_(Z) (1) and Q_(Z) (2) arein the range of 0.3˜0.7.

In the foregoing preferred embodiments, as reflecting electrodes,metallic reflecting electrodes composed of aluminum are used, however,the invention is not limited in this construction. For example, with ametallic reflecting electrode including silver as a component, similarresults can be achieved.

The Fifth Preferred Embodiment

FIG. 10 is a cross section schematically showing the construction of areflective LCD in accordance with the fifth preferred embodiment of thepresent invention. The construction of the display device of thisembodiment is similar to the second preferred embodiment shown in FIG.4, but with no scattering film. Differences from other embodimentsinclude that the scattering type reflecting electrode 48 disposed on theinner surface of the lower substrate 19 is replaced with a transparentelectrode 78, and reflection is performed by a scattering reflectionplate 72 disposed outside of the liquid crystal cell. Due to this, atransparent substrate is used for a lower substrate 79. A silverscattering reflecting plate is used for the scattering reflection plate72 disposed under the lower transparent substrate 79. Other componentsare the same as those of FIG. 4, thus the same elements are denoted withthe corresponding numerals. <The manufacturing method of the reflectiveLCD is not explained here since it is the same as that of the firstpreferred embodiment.

The optical construction of the reflective LCD is the same as FIG. 2.

In this embodiment, the thickness of the liquid crystal is d_(LC)=3.0μm, Δn_(LC)·d_(LC), equal to 0.24 μm, and the twist angle of the liquidcrystal, Ω_(LC)=63.0°. The angle of the absorption axis of thepolarizing film is Ø_(P)=105.0°. As the two retardation films, films ofQ_(Z) =1.0 are used. The retardation value of the retardation film 11 aon the polarizing film side is set as R_(F1)=0.27 μm whereas that of theretardation film 11 b on the liquid crystal cell side is set asR_(F2)=0.14 μm. Angles of the slow axes of the retardation films againstthe absorption axis of the polarizing film are set asØ_(P)−Ø_(F1)=110.0° and Ø_(P)−Ø_(F2)=170.0°. When upper and lowersubstrates are both composed of transparent substrates, and thescattering reflecting plate is used under the lower substrate,insignificant blurring of image is caused by differences in viewingangles, however, a reflective LCD enjoying natural changes in field ofview properties can be achieved.

In terms of the front properties, the reflectance of white converted toa value of Y is 16.8%, and the contrast is 14.5.

Based on the forgoing construction, a reflective LCD is prepared withoutthe color filter layer 73. In this case, as for the front properties,the reflectance of white converted to a value of Y being 33.6%, and thecontrast, 14.1 are achieved.

If an air layer is provided in between the lower substrate and thereflecting plate 72 rather than completely bonding the scatteringreflecting plate 72 with the lower transparent plate 79 with a glue,even more natural field of view properties can be achieved since thediffusion effect increases due to the difference in refractive indexbetween resin (approximately 1.6) and air (1.0).

In this embodiment, a silver scattering reflecting plate is used,however, an aluminum scattering reflecting plate can also achievesimilar effects in. the invention.

The Sixth Preferred Embodiment

FIG. 11 is a cross section schematically showing the construction of areflective LCD in accordance with the sixth preferred embodiment. Theelements having the same function as the first preferred embodimentdescribed in FIG. 1, are denoted with the same numerals.

In this embodiment, a gate electrode 90, a source wire 91, a thin filmtransistor device (TFT) 92, a drain electrode 93 and a flattening film94 are disposed on the lower substrate 19 forming a so called activematrix array. A metallic reflecting electrode 88 is interconnected withthe nonlinear switching device (TFT) 92 under the flattening film 94 viaa contact hole 95. The metallic reflecting electrode 88 is structuredsuch that each pixel is independently separated, therefore each pixel isactively driven by independent signals, thus a high-contrast, highquality display free of cross talk can be achieved.

The optical construction of the reflective LCD of this embodiment is thesame as FIG. 2.

The upper transparent substrate 13 and the lower substrate 19 are madewith non-alkali glass substrate (for example, product 1737 made byCorning Co.). On top of the upper transparent substrate 13 is the colorfilter layer 14, and on top of which is an counter electrode made withan indium tin oxide, formed on the color filter layer 14 as thetransparent electrode. 16.

The gate electrode 90 made with aluminum and tantalum, the sourceelectrode 91 made with titanium and aluminum, and the drain electrode 93are arranged in a matrix form. At each intersection of the gateelectrode 90 and the source electrode 91, a TFT device 92 composed ofamorphous silicon is formed.

Over the entire surface of the lower substrate 19 on which nonlineardevices are formed, a positive photosensitive acrylic resin (forexample, FVR made by Fuji Drag Industry Co.) is coated and theflattering film 94 is formed. Subsequently, using a predetermined photomask, UV lights are irradiated to form the contact hole 95 on the drainelectrode 93. The metallic reflecting electrode 88 of a specularreflection type is then disposed thereupon by vapor-depositing 300nm-thick titanium and, on top of titanium, 200 nm-thick aluminum.

A 5% by weight polyimide y-butyrolactone solution, is printed on thetransparent electrode 16 and the metallic reflecting electrode 88, thencured at 250° C. Subsequently, in an orientating process, a twist of apredetermined angle is provided by a rotation rubbing method using arayon cloth, to form the orientation layers 15 a and 15 b.

The upper transparent substrate 13 and the lower substrate 19 are bondedtogether in a similar method to that of the first preferred embodiment,and the sealing resin is cured at 150° C. Subsequently, a mixture offluorine-ester nematic liquid crystal of the birefringence differenceΔn_(LC)=0.08 to which a predetermined amount of chiral liquid crystal isadded, is injected under vacuum in between the two substrates, and theopening is sealed with ultraviolet curing resin which is then cured byUV lights.

As the scattering film 12, isotropic front scattering film is disposedon the upper transparent substrate 13 of the liquid crystal cell 8formed in the foregoing manner. The retardation films 11 a and 11 b aredisposed on the scattering film 12 in such a manner that their slow axesform an angle specified below. On top of these films, the polarizationfilm 10 is disposed in such a manner that its absorption ortransmittance axis forms an angle specified below.

In this embodiment, the thickness of the liquid crystal is d_(LC)=3.0μm, and Δn_(LC)·d_(LC), equal to 0.24 μm, and the twist angle of theliquid crystal, Ω_(LC)=63.0°. As the two retardation films, films ofQ_(Z)=1.0 are used. The retardation value of the retardation film 11 aon the polarizing film side is set as R_(F1)=0.27 μm whereas that of theretardation film 11 b on the liquid crystal cell side is set asR_(F2)=0.14 m. The angle of the absorption axis of the polarizing filmis Ø_(P)=105.0°. Angles of the slow axes of the retardation filmsagainst the absorption axis of the polarizing film are set asØ_(P)−Ø_(F1)=110.09 and Ø_(P)−Ø_(F2)=170.0°.

The reflective LCD constructed in the foregoing manner achieves a64-gradation, full color display when driven actively. Since theaperture ratio of the pixel is as large as 97% thanks to a metallicelectrode formed on the flattened film, contrast of 15.7, and thereflectance of white converted to a value of Y 19.1% in terms of frontproperties can be achieved.

In all the other embodiments mentioned so far as well, an active-drivereflective LCD can be achieved based on this embodiment by forming anon-linear switching device such as TFT on the lower substrate. As anon-linear switching device, not only amorphous silicon TFT but also twoterminal device such as MIM and thin film diode, and poly-silicon TFTcan be used alternatively for the same effect.

Polycarbonate is used as a retardation film for all the embodiments.This is because, at present, it is preferable in terms of cost andoptical properties. However, the present invention is not limited tothis, and any material which has the same optical anisotropy can be usedas a retardation film.

INDUSTRIAL APPLICABILITY

As thus far described, according to the present invention, anormally-white reflective liquid crystal display device capable ofachromatic black and white display, and bright and high contrastmulti-gradation and multi-color display can be achieved.

Thus, it is possible to provide a liquid crystal display device whichachieves high power efficiency and a high picture quality, therebypromoting the use of liquid crystal display devices in a wide range ofareas including mobile information devices.

What is claimed is:
 1. A reflective liquid crystal display devicecomprising; a) a liquid crystal cell formed by injecting a nematicliquid crystal in between a first and a second substrates; b) apolarizing film disposed on the first substrate of said liquid crystalcell; c) two retardation films disposed in between said polarizing filmand said liquid crystal cell; and d) a light reflecting means disposedon the second substrate, wherein a twist angle of said nematic liquidcrystal Ω_(LC) is 45°˜90°, a product Δ_(LC)·d_(LC) is 0.20 μm˜0.30 μmwhere Δn_(LC) and d _(LC) are respectively birefiringence of saidnematic liquid crystal and thickness of a liquid crystal layer, aretardation value of said retardation film on said polarizing film sideR_(F1) is 0.13 μm˜0.18 μm, a retardation value of said retardation filmon said liquid crystal cell side R_(F2) is 0.13 μm˜0.18 μm, and the Zcoefficient, Q_(Z) of each of said two retardation films is 0.3˜1.0,wherein when a bisector of a larger angle between angles which areformed by orientation directions respectively of liquid crystalmolecules closest to said first substrate and liquid crystal moleculesclosest to said second substrate is set as a reference line inside thesubstrates, and when a direction, in which said nematic liquid crystalis twisted viewed from the first substrate to the second substrate, ispositive, Ø_(P) is 75°˜195°, Ø_(P)−Ø_(F1), 105°˜115°, and Ø_(P)−Ø_(F2),165°˜175°, where Ø_(P) is an angle formed by the reference line and adirection of an absorption axis of said polarizing film, Ø_(F1) is anangle formed by the reference line and a direction of a slow axis ofsaid retardation film on said polarizing film side, and Ø_(F2) is anangle formed by the reference line and a direction of a slow axis ofsaid retardation film on said liquid crystal cell side, and theaforementioned Q_(Z) is a coefficient defined as Q _(Z)=(n _(x) −n_(z))/(n _(x) −n _(y)),  where n_(x), n_(y) and n_(z) are refractiveindices of each retardation film in directions of each axis in spatialcoordinates x, y and z in which the z axis is a direction normal tosurfaces of the retardation film, and n_(x) is a refractive index in adirection of the slow axis and n_(y) an fast axis of the retardationfilm.
 2. The reflective liquid crystal display device of claim 1,wherein the angle Ø_(P), formed by the reference line and the absorptionaxis of said polarizing film, is one of 90°˜120° and 150°˜180°.
 3. Areflective liquid crystal display device comprising; a) a liquid crystalcell formed by injecting a nematic liquid crystal in between a first anda second substrates; b) a polarizing film disposed on said firstsubstrate of said liquid crystal cell; c) two retardation films disposedin between said polarizing film and said liquid crystal cell; and d) alight reflecting means disposed on said second substrate, wherein atwist angle of said nematic liquid crystal Ω_(LC) is 45°˜90°, a productΔn_(LC)·d_(LC) is 0.20 μm˜0.30 μm where Δn_(LC) and d _(LC) arerespectively birefringence of said nematic liquid crystal and thicknessof a liquid crystal layer, a retardation value of said retardation filmon said polarizing film side R_(F1) is 0.13μm˜0.18 μm, a retardationvalue of said retardation film on said liquid crystal cell side R_(F2)is 0.13 μm˜0.18 μm, and the Z coefficient, Q_(Z) of each of said tworetardation film is 0.3˜1.0, wherein when a bisector of a larger of theangle formed by an orientation direction of liquid crystal moleculesclosest to said first substrate and an orientation direction of liquidcrystal molecules closest to said second substrate is set as a referenceline, and when a direction, in which said nematic liquid crystal istwisted from said first substrate to said second substrate, viewed fromthe side of said first substrate, is defined positive, Ø_(P) is−15°˜105°, Ø_(P)−Ø_(F1), −105°˜−115°, and Ø_(P)−Ø_(F2), −165°˜−175°,where Ø_(P) is an angle formed by the reference line and a direction ofan absorption axis of said polarizing film, Ø_(F1) is an angle formed bythe reference line and a direction of a slow axis of said retardationfilm on said polarizing film side, and Ø_(F2) is an angle formed by thereference line and a direction of a slow axis of said retardation filmon said liquid crystal cell side, and the aforementioned Q_(Z) is acoefficient defined as Q _(Z)=(n _(x) −n _(z))/(n _(x) −n _(y)),  wheren_(x), n_(y) and n_(z) are refractive index of each retardation film indirections of each axis in spatial coordinates x, y and z in which the zaxis is a direction normal to surfaces of the retardation films, andn_(x) is a refractive index in a direction of the slow axis and n_(y) anfast axis of the retardation film.
 4. The reflective liquid crystaldisplay device of claim 3, wherein the angle Ø_(P), formed by thereference line and the absorption axis of said polarizing film is one of0°˜30° and 60°˜90°.
 5. The reflective liquid crystal display device ofone of claims 1˜4, wherein the twist angle Ω_(LC) of said nematic liquidcrystal is 60°˜65°.
 6. The reflective liquid crystal display device ofone of claims 1˜4, wherein the Z coefficient, Q_(Z) of said retardationfilm on said polarizing film side is 0.3˜0.7.
 7. The reflective liquidcrystal display device of one of claims 1˜4, wherein the Z coefficient,Q_(Z) of each of said two retardation films is 0.3˜0.7.
 8. Thereflective liquid crystal display device of one of claims 1˜4, furthercomprising a scattering film on said first substrate.
 9. The reflectiveliquid crystal display device of claim 8, wherein said scattering filmis disposed in between said retardation films and the substrate.
 10. Thereflective liquid crystal display device of claim 8, wherein saidscattering film is a front scattering film.
 11. The reflective liquidcrystal display device of one of claims 1˜4, wherein said lightreflecting means is a metallic electrode including at least one ofaluminum and silver as a component.
 12. The reflective liquid crystaldisplay device of claim 11, wherein said metallic electrode has amirror-like surface.
 13. The reflective liquid crystal display device ofclaim 11, wherein a scattering film is disposed on said metallicelectrode.
 14. The reflective liquid crystal display device of claim 11,wherein said metallic electrode has an uneven surface with angles ofslopes being 3°˜12° on average.
 15. The reflective liquid crystaldisplay device of one of claims 1˜4, wherein said second substrate is atransparent substrate, and on outer surface of which said lightreflecting means is disposed.
 16. The reflective liquid crystal displaydevice of one of claim 15, wherein a layer of air is provided betweensaid transparent substrate and said light reflecting means.
 17. Thereflective liquid crystal display device of one of claims 1˜4, wherein acolor filter is disposed on one of the substrates.
 18. The reflectiveliquid crystal display device of one of claims 1˜4, wherein a nonlinearswitching device is disposed on said second substrate.
 19. Thereflective liquid crystal display device of claim 18, wherein ainsulative flattened film is formed on said nonlinear device, andthrough a contact hole formed on said flattened film, said nonlinearswitching device and an electrode on said second substrate side areinterconnected.
 20. The reflective liquid crystal display device of oneof claims 1˜4, wherein said retardation films are made withpolycarbonate.